Plasma magnetron sputtering has been long practiced in the fabrication of silicon integrated circuits. More recently, sputtering has been applied to depositing layers of materials onto large, generally discrete rectangular panels of glass, metal, or polymer or onto equivalent sheets. The completed panel may incorporate thin-film transistors, plasma display, field emitter, liquid crystal display (LCD) elements, or organic light emitting diodes (OLEDs) and is typically directed to flat panel displays. Photovoltaic cells may similarly be fabricated. Related technology may be used for coating glass windows with optical layers. The material of the sputter deposited layer may be a metal, such as aluminum or molybdenum, transparent conductors, such as indium tin oxide (ITO), and yet other materials including silicon, metal nitrides and oxides.
Demaray et al. describe such a flat panel sputter chamber in U.S. Pat. No. 5,565,071, incorporated herein by reference in its entirety. Their sputter chamber 10 includes, as illustrated in the schematic cross section of FIG. 1, a rectangularly shaped sputtering pedestal electrode 12, which is typically electrically grounded, for holding a rectangular glass panel 14 or other substrate in opposition to a rectangular sputtering target assembly 16 within a vacuum chamber 18. The target assembly 16, at least the surface of which is composed of a metal to be sputtered, is vacuum sealed to the vacuum chamber 18 across an isolator 20. Typically, a target layer of the material to be sputtered is bonded to a backing plate in which cooling water channels are formed to cool the target assembly 16. A sputtering gas, typically argon, is supplied into the vacuum chamber 18 held at a pressure in the milliTorr range.
Advantageously, a back chamber 22 or magnet chamber is vacuum sealed to the back of the target assembly 16 and is vacuum pumped to a low pressure, thereby substantially eliminating the pressure differential across the target 16 and its backing plate. Thereby, the target assembly 16 can be made much thinner. When a negative DC bias is applied to the conductive target assembly 16 with respect to the pedestal electrode 12 or other grounded parts of the chamber such as wall shields, the argon is ionized into a plasma. The positive argon ions are attracted to the target assembly 16 and sputter metal atoms from the target layer. The metal atoms are partially directed to the panel 14 and deposit thereon a layer at least partially composed of the target metal. Metal oxide or nitride may be deposited in a process called reactive sputtering by additionally supplying oxygen or nitrogen into the chamber 18 during sputtering of the metal.
To increase the sputtering rate, a magnetron 24 is conventionally placed in back of the target assembly 16. If it has an inner magnetic pole 26 of one vertical magnetic polarity surrounded by an outer magnetic pole 28 of the opposite polarity to project a magnetic field within the chamber 18 and parallel to the front face of the target assembly 16, under the proper chamber conditions a high-density plasma loop is formed in the processing space adjacent the target layer. The two opposed magnetic poles 26, 28 are separated by a substantially constant gap defining the track of the plasma loop. The magnetic field from the magnetron 24 traps electrons and thereby increases the density of the plasma and as a result increases the sputtering rate of the target assembly 16. The relatively small widths of the linear magnetron 24 and of the gap produces a higher magnetic flux density. The closed shape of the magnetic field distribution along a single closed track prevents the plasma from leaking out the ends.
The size of the rectangular panels being sputter deposited is continuing to increase. One generation processes a panel having a size of 1.87 m×2.2 m and is called 40K because its total area is greater than 40,000 cm2. A follow-on generation called 50K has a size of greater than 2 m on each side.
These very large sizes have imposed design problems in the magnetron since the target spans a large area and the magnetron is quite heavy but nonetheless the magnetron should be scanned over the entire area of the target and in close proximity to it.
Tepman addresses many of these problems in U.S. Patent Application Publication 2006/0049040, incorporated herein by reference. In the Tepman design a single large rectangular magnetron having a size only slightly smaller than that of the target is formed with a single inner magnetic pole surrounded by a single outer magnetic pole of the opposite polarity. The gap between them forms a long convolute path defining a closed plasma track adjacent the sputtering face of the target. The magnetron is scanned in a two-dimension pattern extending over dimensions much smaller than those of the magnetron or target. Specifically, the scanning dimension are approximately equal to the pitch between neighboring plasma tracks, thus providing a more uniform sputter erosion of the single continuous target and more uniform sputter deposition. Le et al. describe improvements to the Tepman apparatus and methods of operating it in U.S. patent application Ser. No. 11/484,333, filed Jul. 11, 2006, published as U.S. Published Patent Application 2007/0012562 and incorporated herein by reference.
However, the previously available magnetron sputter chambers for large flat panels have exhibited less than complete target utilization. In particular, the edge portions of the target adjacent the outer periphery of the scanned area of the magnetron are eroded more quickly than interior portions.